Conventionally, a technology of correcting and outputting a luminance (tone) of an input image is widely known. For example, in the technology described in Patent Literature 1 below, first of all, a luminance histogram of an input image is generated. Next, any tone curve of a plurality of tone curves prepared in advance is selected on the basis of this luminance histogram. Then, the luminance of the image is corrected and output on the basis of this tone curve.
In the technology described in Patent Literature 1, with respect to an image whose luminance frequency is concentrated on a middle portion of the luminance histogram, for example, a tone curve having a steep slope in the middle portion of the luminance is used, and luminance tones are preferentially allocated to the middle portion of the luminance. In the technology described in Patent Literature 1, the use of such a tone curve prevents the image from becoming an unclear image having a small difference between brightness and darkness.
In the technology described in Patent Literature 1, a method of preferentially allocating the luminance tones to a part of the input image, in which luminance frequency is high (i.e., portion having the same brightness level and a large area), is employed. However, for example, if such a method is used for an image including the sky (high luminance, large area) and a person (middle luminance, small area), the luminance tones allocated to the sky increase while the luminance tones allocated to the person decrease, and hence a phenomenon in which the contrast for the person is lowered and the person is not clearly visible may occur.
As a technology associated with such a problem, Patent Literature 2 below is exemplified. In this technology, a method of generating a high-frequency component histogram in addition to the luminance histogram and correcting the luminance histogram on the basis of the high-frequency component histogram is employed. In accordance with this method, it is possible to re-distribute tones allocated to the sky corresponding to low-frequency components to the person corresponding to high-frequency components.